Thermal versus acoustic response of velocity sensitive premixed flames
Thermal versus acoustic response of velocity sensitive premixed flames
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DOI：
10.1016/j.proci.2014.07.032
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发表时间：
2015
期刊：
影响因子：
--
通讯作者：
S. Bomberg;T. Emmert;W. Polifke
中科院分区：
文献类型：
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作者：
S. Bomberg;T. Emmert;W. Polifke
预混火焰响应于具有热释放速率波动的速度扰动（“热响应”），这又产生声学扰动（“声学响应”）。后者可能随后影响速度场的反馈导致自激热声不稳定性的方式。本文研究了预混火焰的热响应和声响应之间的相互关系。制定的分析，使它正确地代表了声流火焰声学相互作用的潜在因果关系。揭示了一个火焰固有反馈回路，它与火焰的声环境，即腔室和燃烧室的声阻抗完全无关。该火焰固有反馈回路的本征模与火焰的声散射矩阵的极点相一致。相应的频率，其中的声学响应是最大的，通常是非常不同的频率，其中的热响应是强的，即火焰传递函数表现出“过度增益”。更值得注意的是，固有火焰模式可能会导致热声不稳定性，而不锁定到燃烧室的声学本征模式之一。从两个燃烧室试验台的层流锥形以及湍流旋流火焰的实验结果进行了仔细检查，并证实了我们的分析。特别是，不稳定的模式被确定为与火焰固有反馈密切相关。
Premixed flames respond to velocity perturbations with fluctuations in heat release rate (“thermal response”), which in turn generate acoustic perturbations (“acoustic response”). The latter may subsequently influence the velocity field in such a manner that feedback leads to self-excited thermoacoustic instability. The present paper investigates interrelations between the thermal and the acoustic responses of premix flames. The analysis is formulated such that it properly represents the underlying causality of acoustics–flow–flame–acoustics interactions. A flame-intrinsic feedback loop is revealed, which is quite independent of the acoustic environment of the flame, i.e. the acoustic impedances of plenum and combustor. The eigenmodes of this flame-intrinsic feedback loop coincide with poles of the acoustic scattering matrix of the flame. The corresponding frequencies, where the acoustic response is maximum, are in general quite different from frequencies where the thermal response is strong, i.e. where the flame transfer function exhibits “excess gain”. Even more remarkable, the intrinsic flame modes may result in thermoacoustic instabilities without lock-on to one of the acoustic eigenmodes of the combustor. Experimental results from two combustor test rigs with laminar conical as well as turbulent swirl flames are scrutinized and are found to confirm our analysis. In particular, unstable modes are identified that are strongly related to flame-intrinsic feedback.